The present invention relates to a method and apparatus for forming a mold which is used in casting an article and more specifically to a method and apparatus for positioning mold segments in a desired relationship to one another.
In a known method of forming a mold, mold wall segments are formed by repetitively dipping a wax or plastic pattern assembly in a ceramic slurry and subsequently melting out the pattern to leave a mold cavity. Once the mold wall segments are formed, they are joined to each other to form a complete mold assembly. A method of forming a mold in this manner is shown in U.S. Pat. No. 4,066,116.
In order to avoid deflection of the wax pattern during dipping, the pattern may be reinforced with a reusable, nondestructable pattern piece. A method of reinforcing a wax pattern is shown in U.S. Pat. No. 4,043,379.
When relatively large objects are to be cast with a mold similar to the one shown in U.S. Pat. No. 4,066,116, difficulty has been encountered in obtaining a cast product having the desired dimensions. This may be due to many causes, among which are shrinkage of the wax pattern before dipping, dimensional changes in the ceramic mold during drying and firing, and shrinkage of the casting material upon cooling.
The extent of shrinkage of the cast material is difficult to predict accurately. The amount of shrinkage in any portion of the article depends upon the rate of cooling of that particular portion. In turn the rate of cooling of a portion of a casting is dependent on such factors as the cross sectional area of that portion of the casting and its distance from areas of larger or smaller cross section. It may be possible to calculate the extent of shrinkage for some simple geometric shapes, but for large articles having an annular or tubular shape in which the cross section varies along the axis of the article, it is difficult to accurately predict the extent of shrinkage.
In an effort to provide relatively large and dimensionally accurate castings, a fixture assembly has been utilized to locate sections of a mold relative to each other. This known fixture assembly is constructed in the manner disclosed in U.S. Pat. No. 4,224,976. This application discloses a fixture having a plurality of upstanding pin members which are utilized to support a circular array of mold wall sections. The positions of the pin members can be varied to adjust the diameter of the circular array of mold wall sections.
Although the fixture disclosed in the aforementioned U.S. Pat. No. 4,224,976 is preferred to other known devices for use in assembling molds, difficulty may be encountered in using this fixture when a relatively large article having portions with different cross sectional configurations is to be cast. For example, when an annular article having different configurations along the axial extent of the article is to be cast, it may be desirable to adjust the diameters of opposite axial ends of the annular mold to different extents. With the device disclosed in U.S. Pat. No. 4,224,976 both axial ends of the mold are adjusted to the same extent when the positions of the upstanding pin members are changed.
A mold which is utilized to cast annular articles may have inner and outer arrays of mold wall segments which are interconnected in the manner disclosed in U.S. Pat. No. 4,066,116. When a mold is assembled of inner and outer arrays of mold wall segments, it may be desirable to move both the inner and outer mold wall segments relative to each other. In this situation, difficulty may also be encountered in using the apparatus disclosed in the aforementioned U.S. Pat. No. 4,224,976. Because the fixture disclosed in this application does not provide for movement of a mold wall segment in an annular outer array of mold wall segments relative to a mold wall segment in an annular inner array of mold wall segments.